The invention relates to a method and apparatus for distinguishing regions where a material is present on a surface from other regions, as set forth in the preamble of claims 1 and 5.
Such a method and apparatus are known from PCT patent application no WO 98/2756.
This document concerns the inspection of a printed circuit board (PCB) assembly. A PCB assembly contains a substrate and conductor tracks on the substrate. During manufacture solder paste is applied to the conductor tracks and leads of components are soldered onto the conductor tracks.
The manufacturing process of the PCB assembly is supported by measurements, for purposes such as alignment and inspection. PCT patent publication WO 98/2756 describes optical measurement of the height of the PCB assembly as a function of position of xy-position on the substrate surface. The height at positions where there are conductor tracks is greater than at positions where there is only substrate. The height is used to distinguish the positions where conductor tracks are present on the surface from other positions. Similarly the position of components and of solder paste can be determined. This can be used for example for inspection, for measuring the position of conductor tracks more accurately (e.g. for component placement) and/or for computing the area or volume of solder paste or conductor tracks. Instead of height measurement other properties of the PCB assembly may be measured, for example light reflection or absorption etc.
It has been found that the measurement of height and of other properties suffers from noise. There is a spread in the measured height values which sometimes makes it difficult to distinguish regions with the height level of conductor tracks from regions with the height level of the substrate. Similarly it may be difficult to detect components or solder paste. In general the noise reduces the reliability with which the height level may be used to distinguish regions with different materials on the substrate surface.
It is an object of the invention to increase the reliability with which a region where a material is present on a surface may be distinguished.
It is a further object of the invention to increase the accuracy with which the location of a region where a material is present on a surface may be determined.
The method and apparatus according to the invention are characterized as set. The invention is based on the realization that the nature of the statistical spread of a sensed properties of the surface such as the height is different for different materials on the surface. Hence, a measure of the spread in the measurement of such a property may be computed and used to distinguish regions where a certain material is present. Such a measure of spread could be for example the variance (mean square deviation) of the height, the standard deviation or some measure of correlation between the heights at different positions.
In a PCB assembly for example there is less mean square deviation in the measured height of the conductor tracks than in the measured height of the substrate where there are no conductor tracks. When the conductor tracks are covered by solder paste the means square deviation is even lower.
Therefore in an embodiment of the invention the method and apparatus are used for distinguishing regions on a PCB assembly where conductor tracks are respectively present or absent by distinguishing regions where the spread is relatively smaller or larger respectively. Preferably the sensed property is representative of height of material above the surface. The method of sensing this property may for example be optical e.g. sensing the position of intersection between a scanning light source and the surface or the material on the surface.
In an embodiment of the method and apparatus according to the invention the spread is determined in each of a series of windows of positions on the surface.